THF hydrate

Ross et al. (1981) also determined that the THF hydrate thermal conductivity was proportionally dependent on temperatures, but had no pressure dependence. 

Water mobility from molecular reorientation and diffusion. Evidence for the motion of the water molecules in crystal structures is typically provided by XH NMR (Davidson and Ripmeester, 1984). At very low temperatures (<50 K) molecular motion is frozen in so that hydrate lattices become rigid and the hydrate proton NMR analysis suggests that the first-order contribution to motion is due to reorientation of water molecules in the structure the second-order contribution is due to translational diffusion. 2H NMR has been also used to measure the reori-entational rates of water and guest molecules in THF hydrate (Bach-Verges et al., 2001). Spin lattice relaxation rates (fy) have been measured during THF hydrate... 

Structure identification and relative cage occupancies. The hydration number and relative cage occupation for pure components and guests were measured by Sum et al. (1997), Uchida et al. (1999), and Wilson et al. (2002). Raman guest spectra of clathrate hydrates have been measured for the three known hydrate crystal structures si, sll, and sH. Long (1994) previously measured the kinetic phenomena for THF hydrate. Thermodynamic sl/sll structural transitions have been studied for binary hydrate systems (Subramanian et al., 2000 Schicks et al., 2006). 

Papadimitriou, N.I., Tsimpanogiannis, I.N., Papaioannou, A.Th., and Stubos, A.K. (2008) Evaluation of the hydrogen-storage capacity of pure H2 and binay H2-THF hydrates with Monte Carlo simulations. /. Chem. Phys. C, 112,... 

Figure 3 (Left) Calorimetric and dielectric relaxation time of pure and KOH-doped THF hydrate crystal. (Right) A Cole-Cole plot of KOH-doped THF hydrate crystal.

The situation is quite similar to that of ice. A dielectric measurement on a KOH-doped THE showed that the relaxation time t for the reorientational motion was dramatically shortened by the dopant, possibly by creating a pair of the orientational defects proposed by Bjemim. Not only the absolute value of t but also the activation energy for the process decreased by the dopant, as shown in Fig. 3. The value of x at 70 K is 10- times smaller than that for pure (undoped) sample. This is the reason why the ordering transition has escaped from observation for a pure sample by a kinetic reason appeared now at 62 K in the doped sample by a catalytic action of the dopant within a reasonable time. Also given in the figure is a Cole-Cole plot of the dielectric permittivity of the KOH-doped THF hydrate. The distribution of dielectric relaxation times is much wider in the doped sample than in the pure sample. 

A second example comes from the reaction of Xe with powdered THF hydrate . It is easy to see that a hydrate of sll forms quickly on the surface as it is not necessary to form to nucleate a new phase. Since the activity of the Xe guest is much greater than that of THF under the initial experimental conditions, Xe replaces THF in the surface layers of the hydrate. This metastable hydrate persists for several hundred seconds before it converts completely to si Xe hydrate (which does have to nucleate) plus a mixed THF-Xe hydrate. Such experiments reveal that hydrates are extremely labile at temperatures above 200 C, and that metastable hydrates may be important in the early stages of hydrate formation. 

Figure 5 Crystal morphology assays. Formed ice crystals (1-3) were examined under a microscope (40x). Representative crystals were grown in the presence of (1) control solutions such as buffer (shown), E. coli or medium, (2) Type I fish AFP and (3) P. borealis cultures. THF hydrate crystals (4-6) were grown at a pipette tip for about the same amount of time and transferred to solutions of (4) THF solution (15 1 molar ratio), (5) THF.solution containing PVP and (6) THF solution containing Type I AFP. The bar represents 1.3 mm.

The addition of THF induces the large pressure reduction from the equilibrium pressure without THF. The largest pressure reduction is obtained when the THF mole fraction is 0.056, which is the stoichiometric mole fraction for the pure THF hydrate. 

Carlo simulation of ice-crystal growth to study the mechanism of inhibition of AFPs on the surface of the THF hydrate and propane hydrate. They found that most of the octahedron surfaces of the THF hydrate were covered with AFP molecules, which could reduce the growth rate of the THF hydrate only allowing plate growth perpendicular to that surface. It is thus necessary to look for other experimental evidences to clarify the common features of AFPs on the inhibition of the clathrate hydrate formation. 

Although AF(G)Ps and LDHIs are distinct, they both inhibit the growth of crystals. Neither AFGP nor PVP are reported to significantly affect ice nucleation, and similary, we have shown that AFPs and PVP did not affect homogeneous nucleation of THF hydrate. It is not known if these two types of inhibitors can adsorb to other hydrophilic surfaces, however silica is an ubiquitous impurity and common to both these systems. Thus, it is of interest to determine the effects of these inhibitors on heterogeneous nucleation of ice/clathrate hydrate. 

Xiaobing Lu, Li Wang, Shuyun Wang, et al. 2007. Experimental study of mechanical properties of THF hydrate-bearing sediment. Proceeding of the Thirteenth China Ocean Engineering, 681-684. 

Lu X.B., Wang L., Wang S.Y. et al. 2008. Study on the mechanical properties of THF hydrate deposit. Proc. 18th Int. Offshore and Polar Engrg. Conf, Vancouver, 57-60. 

The nucleation and growth of hydrates on ice surfaces have been studied using hyperpolarised Xe NMR spectroscopy. Xe NMR spectroscopy has been used to study diffusion in water-saturated porous media. The temperature dependence of the hyper-polarised Xe NMR signal has been used to study the interconversion of species in ice and THF hydrate. ... 

Figure 7 Scaled thermal conductivities for ice Ih, THF hydrate (THFW), and six amorphous solids PB, polybutadiene PET, poly(ethylene terephthalate) PS, polystyrene PMMA, poly(methyl methacrylate). The solid curve is a least squares fit to the ice experimental data. Reprinted (adapted) with permission from J. Phys. Chem., 92,5006 (1988). Copyright 1998 American Chemical Society.

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